1. Field of the Invention
This invention relates generally to the field of cable systems for video cameras. More particularly, this invention relates to a cable system that is compatible with both analog signals transmitted via triaxial electrical conductors and with digital video signals transmitted via optical fibers.
2. Description of Related Art
The introduction of digital techniques for capturing and editing video images has lead to improvements in the quality of images available to consumers and to greater flexibility for editors and producers.
Digital video cameras that capture images using arrays of pixels and compile those pixels into a video signal represent the state of the art in video camera technology. These cameras are rugged, compact, and deliver high-quality video images. The vast majority of video cameras used by consumers and professionals incorporate a digital image capturing mechanism.
Digital editing systems are also well known and represent the state of the art in professional video production systems. Editing video signals in the digital domain allows the use of sophisticated computerized techniques. Digital images may be seamlessly enhanced and combined to produce programs that would be impossible to produce using analog systems.
Optical fibers may be used to transmit a digital video signal from a digital camera to an editing system. The digital image signal from the camera is converted to an encoded train of light pulses by a camera adapter using a loss-less conversion process. These pulses are directed along an optical fiber to a camera control unit. The camera control unit converts the encoded pulse train into an exact replica of the original digital signal and provides this signal to editing, recording and broadcasting equipment.
An obstacle to the use of optical fiber cables is that much of the equipment currently used by professional broadcasters is compatible with triaxial cabling using a standard "kings" type connector. In order to accommodate this equipment most broadcast venues, for example sports stadiums, are wired with triaxial cabling and standard connectors. Standardized cabling simplifies the task of setting up cameras and editing facilities at such venues, since broadcasters simply need to connect cameras and editing systems via the installed triaxial cable. It is expected that for the foreseeable future venue owners will continue to install triaxial cable.
Triaxial electrical cable has a narrower bandwidth than optical fiber. As a result digital video signals cannot reliably be transmitted along triaxial cable using loss-less compression techniques. Rather than compress the digital signal, most systems convert the digital signal to an analog signal and then transmit the analog signal along the triaxial cable. The received signal is then reconverted to a digital signal.
The conversion of a digital signal to an analog signal and then from that analog signal back to a digital signal necessarily results in a loss of image quality. Digital video images captured by a camera are a collection of quantized light intensity values for each pixel of the video frame. Conversion of this digital signal to an analog signal requires that voltage levels between the pixel quantizations, as well as the signal values between pixels, be interpolated to produce a continuous analog signal. The analog signal is transmitted along a cable and is received by, for example, a digital editing system.
The analog signal is then sampled and quantized to convert it back to the digital domain. The sample points of the converted signal will not necessarily coincide with the positions of the original pixels captured by the camera and the quantization of the interpolated signal will therefore be an approximation of the original digital signal. This results in a loss of image fidelity between the captured image and the reconverted digital signal.
It is therefore advantageous for the captured digital image to be transmitted from the camera to the editing system completely in the digital domain along cables that are compatible with existing video equipment.